DESCRIPTION (Verbatim from Applicant's Abstract): Virus particles perform a number of essential functions during the viral replication cycle. they provide a stable container that protects the genome during transport form cell to cell, they recognize receptor molecules on the surface of suitable target cells, they penetrate cellular membranes and release the genome in a biologically active form. These functions are performed efficiently and with high precision raising a question central to modern virology: how are capsid functions regulated at the molecular level? The ability of viral capsids to perform their functions is encoded in the chemical and physical properties of their components. In this application we plan to focus on the role of nucleic acid in viral assembly and its impact on the structural and functional properties of the virion. To this end, we will use the highly developed experimental model system represented by the icosahedral RNA containing nodaviruses. Viral genomes have traditionally been considered to be relatively passive components unless they are engaged in processes that reflect their roles as carriers of genetic information, e.g., transcription and replication. There is increasing evidence, however, that this assumption does not reflect reality and that at least in some viruses nucleic acid plays a much more significant part in functions commonly associated with the capsid protein. Results that we obtained in the previous funding period indicate that the packaged nod viral genome restricts the dynamic properties of the virion and that its replacement with heterologous RNA leads to aberrations during viral assembly. These observations suggest that coat protein and viral RNA establish a functional dialogue that is maintained throughout the lifespan of the virion. Understanding this dialogue and recognizing its broader implications is not only of intrinsic importance to virus research but also required for the successful pursuit of practical applications involving virus particles. In this proposal we will continue to take full advantage of the unique opportunities afforded by the nodaviral model system. In the experiments outlined below we intend to elucidate the molecular details of the initial interactions between coat protein and RNA, investigate the mutual dependency of the two components during assembly and analyze how coat protein-RNA interactions manifest themselves in the physical properties of the assembled particle. Results from these studies should be applicable in general terms to medically important positive strand RNA viruses.